The printing plate precursor for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a so-called processless printing plate precursor has been noticed which can be applied to a printing press employing a direct imaging (DI) process and does not require development by a special developing agent.
In the processless printing plate precursor, the same grained aluminum plate as in a PS plate is considered to be used as a hydrophilic substrate, but various printing plate precursors comprising a substrate and a hydrophilic layer coated thereon have been proposed in view of freedom of layer constitution and cost reduction.
As one of the image formation methods employing a processless printing plate precursor, an infrared laser image formation method is preferred, and the methods are generally classified into three methods, an ablation method described later, a heat fusion image formation on press development method and a heat melt image transfer method.
The infrared laser image formation method enables forming an image with high resolution in a short time, but has problem in that an exposure device used is expensive.
As the ablation methods, there are methods, as disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773. These references disclose a printing plate precursor comprising a substrate and a hydrophilic layer or a lipophilic layer as an outermost layer. In the printing plate precursor having a hydrophilic layer as an outermost layer, the hydrophilic layer is imagewise exposed to imagewise ablate the hydrophilic layer, whereby the lipophilic layer is exposed to form image portions. However, there are problem that contamination of the interior of the exposing apparatus by scattered matter caused by the ablation of the surface layer. In order to prevent such a scattered matter caused by the ablation of the surface layer, there is a method in which a water soluble protective layer is further provided on the hydrophilic layer and then the ablated layer is removed together with the protective layer on the printing press.
As the heat fusion image formation on press development method, there is a method employing a printing plate precursor, disclosed in, for example, Japanese Patent Nos. 2938397 and 2938398, which comprises an image forming layer containing thermoplastic polymer particles and a water soluble binder on the hydrophilic layer or on a grained aluminum plate. However, such a printing plate, when a grained aluminum plate is used as a hydrophilic substrate, is required to contain a light heat conversion material (generally colored) in the image forming layer, and may contaminate a printing press in development on press. In this case, a printing plate precursor is advantageous which comprises a substrate and provided thereon, a hydrophilic layer containing a light heat conversion material and an image forming layer containing no light heat conversion material.
As the heat melt image transfer method, there is a method in which a heat melt material is imagewise transferred onto a hydrophilic surface of a metal sleeve which can be repeatedly used, not on a grained aluminum surface, through a heat transfer ribbon such as DICO web produced by Mannroland Co., Ltd., and heated to fix the resulting image.
As described above, any type image recording methods use a plate precursor having a hydrophilic layer as a water receiving surface at non-exposed portions as a processless printing plate for infrared recording.
As an image formation method employing the processless printing plate precursor, there have been various proposed methods including an ink jet recording method. The ink jet recording method does not provide a high speed recording or high resolution obtained by an infrared recording, but has advantage in cheapness.
A processless plate employing an ink jet recording process is a printing plate manufactured by imagewise providing an oleophilic image formation material onto a hydrophilic layer of a plate precursor to form an image and optionally carrying out fixing treatment or treatment for increasing strength of the formed image. As ink containing an image formation material, there is a photopolymerizable ink disclosed in Japanese Patent O.P.I. Publication No. 5-204138, a hot melt ink disclosed in Japanese Patent O.P.I. Publication No. 9-58144, or an oily ink disclosed in Japanese Patent O.P.I. Publication Nos. 10-272753. As a printing plate precursor having a hydrophilic surface, there is a grained aluminum plate or a printing plate precursor having a hydrophilic layer on the support. A printing plate precursor is advantageous for the ink jet recording process which comprises a porous hydrophilic layer, since a suitable ink absorption property is required for a printing plate precursor to prevent ink provided on the printing plate precursor from bleeding or to properly fix the provided ink.
As a processless plate employing various image formation methods, a printing plate precursor having a hydrophilic layer coated on a support is more suitable than that having a grained aluminum plate.
Various studies on a grained aluminum plate have been made, and as a result, an aluminum plate providing excellent printing properties has been obtained. However, although many studies on a hydrophilic layer have been made, a hydrophilic layer providing an excellent performance identical to the grained aluminum plate has not yet been obtained.
The grained aluminum plate has a multiple roughness structure from sub microns to scores of microns, which increases latitude of dampening water ink balance during printing and prevents printed images from abrading to improve printing durability. Proposal has been made which provides a specific shape to a hydrophilic layer. For example, Japanese Patent No. 3019366 discloses a hydrophilic layer having a specific surface roughness and waviness index. Japanese Patent O.P.I. Publication No. 9-99662 discloses a hydrophilic layer with a three dimensional network structure and a void content of from 30 to 80% which contains inorganic particles with a particle size of not more than 100 nm and a water soluble resin. However, these hydrophilic layers have a simple surface shape, and inferior to the grained aluminum plate. Thus, it has been difficult to give a multiple surface roughness to a hydrophilic layer according to the prior art.